Apparatus and circuits including transistors with different gate stack materials and methods of fabricating the same

ABSTRACT

Apparatus and circuits including transistors with different gate stack materials and methods of fabricating the same are disclosed. In one example, a semiconductor structure is disclosed. The semiconductor structure includes: a substrate; a channel layer formed over the substrate; a first transistor formed over the channel layer, wherein the first transistor comprises a first source region, a first drain region, a first gate structure, and a first polarization modulation portion under the first gate structure; and a second transistor formed over the channel layer, wherein the second transistor comprises a second source region, a second drain region, a second gate structure, and a second polarization modulation portion under the second gate structure, wherein the first polarization modulation portion is made of a material different from that of the second polarization modulation portion.

RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/753,530, entitled “APPARATUS AND CIRCUITS INCLUDING TRANSISTORSWITH DIFFERENT GATE STACK MATERIALS AND METHODS OF FABRICATING THESAME,” and filed on Oct. 31, 2018, the entirety of which is incorporatedby reference herein.

BACKGROUND

In an integrated circuit (IC), an enhancement-mode N-type transistor,e.g. enhancement-mode high-electron-mobility transistor (E-HEMT), may beused as a pull-up device to minimize static current. In order to achievenear full-rail pull-up voltage and fast slew rate, a significantly largeover-drive voltage is needed for an N-Type enhancement-mode transistor.That is, the voltage difference between gate and source (Vgs) should bemuch larger than the threshold voltage (Vt), i.e. (Vgs−Vt>>0). It isimperative to use a multi-stage E-HEMT based driver for integratedcircuit to minimize static current. Nevertheless, multi-stage E-HEMTbased drivers will not have enough over-drive voltage (especially forthe last-stage driver) due to one Vt drop across each stage of E-HEMTpull-up device and one forward voltage (Vf) drop across boot-strapdiode. Although one can reduce the Vt for the pull-up E-HEMT transistorsand Vf of diode-connected E-HEMT rectifier of multi-stage drivers toprovide significantly enough over-drive voltage and dramatically reducestatic current, the noise immunity will be compromised.

In an existing semiconductor wafer, transistors formed on the wafer haveidentical structure such that they have a same threshold voltage Vt.When Vt of one transistor is reduced, Vt's of other transistors on thewafer are reduced accordingly. As Vt being reduced in this case, a powerswitch HEMT driven by the HEMT-based driver will have a poor noiseimmunity because the power switch HEMT cannot withstand a largeback-feed-through impulse voltage to its gate. Thus, existing apparatusand circuits including multiple transistors are not entirelysatisfactory.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that various features are not necessarily drawn to scale. In fact,the dimensions and geometries of the various features may be arbitrarilyincreased or reduced for clarity of discussion. Like reference numeralsdenote like features throughout specification and drawings.

FIG. 1 illustrates an exemplary circuit having a multi-stageboot-strapped driver, in accordance with some embodiments of the presentdisclosure.

FIG. 2 illustrates a cross-sectional view of an exemplary semiconductordevice including transistors with different gate stack materials, inaccordance with some embodiments of the present disclosure.

FIGS. 3A, 3B, 3C, 3D, 3E, 3F, 3G, 3H, 3I, 3J, 3K, 3L, 3M, 3N, 3O and 3Pillustrate cross-sectional views of an exemplary semiconductor deviceduring various fabrication stages, in accordance with some embodimentsof the present disclosure.

FIG. 4A and FIG. 4B show a flow chart illustrating an exemplary methodfor forming a semiconductor device including transistors with differentgate stack materials, in accordance with some embodiments of the presentdisclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following disclosure describes various exemplary embodiments forimplementing different features of the subject matter. Specific examplesof components and arrangements are described below to simplify thepresent disclosure. These are, of course, merely examples and are notintended to be limiting. For example, the formation of a first featureover or on a second feature in the description that follows may includeembodiments in which the first and second features are formed in directcontact, and may also include embodiments in which additional featuresmay be formed between the first and second features, such that the firstand second features may not be in direct contact. In addition, thepresent disclosure may repeat reference numerals and/or letters in thevarious examples. This repetition is for the purpose of simplicity andclarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. Terms such as“attached,” “affixed,” “connected” and “interconnected,” refer to arelationship wherein structures are secured or attached to one anothereither directly or indirectly through intervening structures, as well asboth movable or rigid attachments or relationships, unless expresslydescribed otherwise.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

Reference will now be made in detail to the present embodiments of thedisclosure, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers are used in thedrawings and the description to refer to the same or like parts.

An enhancement-mode high-electron-mobility transistor (HEMT), e.g. agallium nitride (GaN) HEMT, has superior characteristics to enable highperformance and smaller Ruin factor in power conversion and radiofrequency power amplifier and power switch applications compared tosilicon based transistors. But there is no viable p-type HEMT availablemostly due to much lower p-type mobility and partly due to twodimensional hole gas (2DHG) band structure. While n-type GaN HEMTs areused in an integrated circuit, to minimize static current, the pull-updevices are mostly based on enhancement-mode n-type transistors ratherthan depletion-mode n-type transistors.

A multi-stage HEMT based driver can be used for an integrated circuit tominimize static current. But multi-stage HEMT based drivers will nothave enough over-drive voltage (especially for the last-stage driver)due to one threshold voltage (Vt) drop across each stage of HEMT pull-updevice and one forward voltage (Vf) drop across boot-strap diode.Although one can reduce the Vt for the pull-up HEMT transistors and Vfof diode-connected HEMT rectifier of multi-stage drivers to providesignificantly enough over-drive voltage and dramatically reduce staticcurrent, the noise immunity will be compromised.

Instead of reducing a single value of the threshold voltage (Vt) of theHEMT transistors in an IC, the present teaching discloses apparatus andcircuits including dual-Vt transistors and their fabrication process. Inone embodiment, two transistors formed on a same wafer have differentVt's. In particular, two transistors have different gate stack materialsfor their gates respectively to obtain different Vt's from each other. AGaN based transistor may have a gate disposed on a gate stack layerincluding p-type doped GaN (pGaN). Various p-type doping materials ofthe GaN gate stack layer may be chosen to obtain various Vt transistorscorrespondingly. Several column I and column II elements, e.g. magnesium(Mg), lithium (Li), sodium (Na), beryllium (Be), calcium (Ca), can bechosen as doping materials on the GaN gate stack layer for the GaN basedtransistors. Different p-type doping materials will induce differentwork functions of the pGaN gate to achieve different-Vt GaN devices. Forinstance, a Mg-doped pGaN layer will induce a higher Vt than a Be-dopedpGaN layer. In an exemplary method of fabricating the dual-Vttransistors, two pGaN gate stacks can be formed by depositing andpolishing GaN with different dopant materials.

The disclosed apparatus can adjust the work function differences in themetal-gates and the pGaN gate stacks to create dual-Vt (or various-Vt)transistors on a same semiconductor wafer; and generate different amountof 2-Dimensional Electron Gas (2-DEG) for transistors at differentlocations of the same wafer at thermal equilibrium.

The present disclosure is applicable to any transistor based IC. Theproposed apparatus and methods can enable a transistor based IC toreduce the static current significantly and have significantly largeover-drive voltages for drivers of concern; without compromising noiseimmunity while increasing over-drive voltages and reducing staticcurrents. In addition, the disclosed apparatus and methods can provideIC designers the flexibility of using different Vt devices for specificfunctions of improving performance, reducing static current, improvingnoise immunity, etc.

FIG. 1 illustrates an exemplary circuit 100 having a multi-stageboot-strapped driver, in accordance with some embodiments of the presentdisclosure. As shown in FIG. 1, the circuit 100 includes a driver havingmultiple stages 110, 120, 130 serially connected to drive a power switchHEMT 175. Each stage includes multiple transistors.

The stage 110 in this example includes transistors 141, 151, 152, 153,154, 155, 156. In one embodiment, among these transistors, thetransistor 154 is a low voltage depletion-mode high electron mobilitytransistor (LV D-HEMT) 192; while each of the other transistors 141,151, 152, 153, 155, 156 is a low voltage enhancement-mode high electronmobility transistor (LV E-HEMT) 191.

As shown in FIG. 1, the gate of the transistor 151 is electricallyconnected to an input pin 131 of the circuit 100. The input pin 131 hasan input voltage Vin ranged from a low logic state voltage (e.g. 0 V) toa high logic state voltage (e.g. 6V). When the circuit 100 is turnedoff, the Vin is 0. The circuit 100 is turned on after the Vin isincreased to 6V. The transistor 151 has a source electrically connectedto ground Vss 111 which has a ground voltage 0V; and has a drainelectrically connected to a source of the transistor 154. The transistor152 in this example has a gate electrically connected to the input pin131, a source electrically connected to the ground Vss 111 which has aground voltage 0V, and a drain electrically connected to a source of thetransistor 155. Similarly, the transistor 153 in this example has a gateelectrically connected to the input pin 131, a source electricallyconnected to the ground Vss 111 which has a ground voltage 0V, and adrain electrically connected to a source of the transistor 156.

The transistor 154 in this example has a gate electrically connected toits own source, which is electrically connected to the drain of thetransistor 151. Drain of the transistor 154 is electrically connected toa source of the transistor 141. The transistor 155 in this example has agate electrically connected to the source of the transistor 154 andelectrically connected to the drain of the transistor 151. Thetransistor 155 has a source electrically connected to the drain of thetransistor 152, and a drain electrically connected to a power supply pinVDD 101 which has a positive power supply voltage (e.g. 6V). Similarly,the transistor 156 in this example has a gate electrically connected tothe source of the transistor 154 and electrically connected to the drainof the transistor 151, a source electrically connected to the drain ofthe transistor 153, and a drain electrically connected to the powersupply pin VDD 101 which has a positive power supply voltage 6V.

The transistor 141 in this example has a gate electrically connected toits own drain, which is electrically connected to the power supply pinVDD 101 which has a positive power supply voltage 6V. The transistor 141connected in this specific configuration is functioning like a rectifieror diode and is conventionally called as a diode-connected transistor.Source of the transistor 141 is electrically connected to the drain ofthe transistor 154. The stage 110 further includes a capacitor 121coupled between the source of the transistor 141 and the source of thetransistor 155.

The stage 120 in this example includes transistors 142, 161, 162, 163,164, 165, 166. In one embodiment, among these transistors, thetransistor 164 is a low voltage depletion-mode high electron mobilitytransistor (LV D-HEMT) 192; while each of the other transistors 142,161, 162, 163, 165, 166 is a low voltage enhancement-mode high electronmobility transistor (LV E-HEMT) 191.

As shown in FIG. 1, the gate of the transistor 161 is electricallyconnected to a node 181, which is electrically connected to the sourceof the transistor 156 and the drain of the transistor 153. The node 181has a voltage ranged between Vss and VDD (0 and 6V). When the circuit100 is turned off, the Vin is 0, such that the transistor 153 is turnedoff and the transistor 156 is turned on. The node 181 has the samevoltage 6V as the power supply pin VDD 101. When the circuit 100 isturned on and the Vin has a voltage of 6V, the transistor 153 is turnedon and the transistor 156 is turned off. The node 181 has the samevoltage 0V as the ground Vss 111.

The transistor 161 has a source electrically connected to ground Vss 111which has a ground voltage 0V; and has a drain electrically connected toa source of the transistor 164. The transistor 162 in this example has agate electrically connected to the node 181, a source electricallyconnected to the ground Vss 111 which has a ground voltage 0V, and adrain electrically connected to a source of the transistor 165.Similarly, the transistor 163 in this example has a gate electricallyconnected to the node 181, a source electrically connected to the groundVss 111 which has a ground voltage 0V, and a drain electricallyconnected to a source of the transistor 166.

The transistor 164 in this example has a gate electrically connected toits own source, which is electrically connected to the drain of thetransistor 161. Drain of the transistor 164 is electrically connected toa source of the transistor 142. The transistor 165 in this example has agate electrically connected to a node 185, which is electricallyconnected to the source of the transistor 164 and electrically connectedto the drain of the transistor 161. The transistor 165 has a sourceelectrically connected to the drain of the transistor 162, and a drainelectrically connected to the source of the transistor 142. Thetransistor 166 in this example has a gate electrically connected to anode 186, which is electrically connected to the source of thetransistor 165 and electrically connected to the drain of the transistor162, a source electrically connected to the drain of the transistor 163,and a drain electrically connected to a power supply pin VDD 102 whichhas a positive power supply voltage (e.g. 6V).

The transistor 142 in this example has a gate electrically connected toits own drain (i.e. diode-connected to act like a rectifier or diode),which is electrically connected to the power supply pin VDD 102 whichhas a positive power supply voltage 6V. Source of the transistor 142 iselectrically connected to the drain of the transistor 164 and the drainof the transistor 165. The stage 120 further includes a capacitor 122coupled between a node 184 electrically connected to the source of thetransistor 142 and a node 183 electrically connected to the source ofthe transistor 166.

The stage 130 in this example includes transistors 143, 171, 172, 173,174. In one embodiment, each of these transistors is a low voltageenhancement-mode high electron mobility transistor (LV E-HEMT) 191. Asshown in FIG. 1, the gate of the transistor 171 is electricallyconnected to a node 182, which is electrically connected to the node181, the source of the transistor 156 and the drain of the transistor153. Same as the node 181, the node 182 has a voltage ranged between Vssand VDD (0 and 6V). When the circuit 100 is turned off, the Vin is 0,such that the transistor 153 is turned off and the transistor 156 isturned on. The node 181 and the node 182 have the same voltage 6V as thepower supply pin VDD 101. When the circuit 100 is turned on and the Vinhas a voltage of 6V, the transistor 153 is turned on and the transistor156 is turned off. The node 181 and the node 182 have the same voltage0V as the ground Vss 111.

The transistor 171 has a source electrically connected to ground Vss 111which has a ground voltage 0V; and has a drain electrically connected toa source of the transistor 173. The transistor 172 in this example has agate electrically connected to the node 182, a source electricallyconnected to the ground Vss 111 which has a ground voltage 0V, and adrain electrically connected to a source of the transistor 174.

The transistor 173 in this example has a gate electrically connected tothe node 186, which is electrically connected to the source of thetransistor 165. The transistor 173 has a source electrically connectedto the drain of the transistor 171, and a drain electrically connectedto a source of the transistor 143. The transistor 174 in this examplehas a gate electrically connected to a node 187, which is electricallyconnected to the source of the transistor 173 and electrically connectedto the drain of the transistor 171. The transistor 174 has a sourceelectrically connected to the drain of the transistor 172, and a drainelectrically connected to a power supply pin VDD 103 which has apositive power supply voltage (e.g. 6V).

The transistor 143 in this example has a gate electrically connected toits own drain (i.e. diode-connected to act like a rectifier or diode),which is electrically connected to the power supply pin VDD 103 whichhas a positive power supply voltage 6V. Source of the transistor 143 iselectrically connected to the drain of the transistor 173. The stage 130further includes a capacitor 123 coupled between a node 189 electricallyconnected to the source of the transistor 143 and a node 188electrically connected to the source of the transistor 174.

As such, the stages 110, 120, 130 are serially connected to form amulti-stage driver that drives a power switch transistor 175. In oneembodiment, the power switch HEMT 175 is a high voltage enhancement-modehigh electron mobility transistor (HV E-HEMT) 193. As shown in FIG. 1,the power switch HEMT 175 has a gate electrically connected to the node188 (whose voltage can be monitored or controlled via pin 132), a sourceelectrically connected to ground Vss 112 which has a ground voltage 0V,and a drain electrically connected to an output pin 133 of the circuit100. In some embodiments, the circuit 100 can serve as a low-side driverin a half-bridge or full-bridge power converter, where the output pin133 serves as a low-side voltage output (LoVout).

Most transistors in FIG. 1 are enhancement-mode N-type transistors. Thatis, the circuit 100 uses mostly enhancement-mode N-type transistors aspull-up devices to minimize static current. In order to achieve nearfull-rail pull-up voltage and fast slew rate, a significantly largeover-drive voltage is needed for the N-Type enhancement-mode transistor.That is, the voltage difference between gate and source (Vgs) should bemuch larger than the threshold voltage (Vt), i.e. (Vgs−Vt>>0). While themulti-stage driver of the circuit 100 can minimize static current, eachstage of E-HEMT pull-up device consumes at least one Vt voltage drop.

As discussed above, the node 181 has a voltage ranged between Vss andVDD (0 and 6V). When the circuit 100 is turned off, the Vin is 0, suchthat the transistor 153 is turned off and the transistor 156 is turnedon. The node 181 has the same voltage 6V as the power supply pin VDD101, which enables the transistors 161, 162, 163 to be turned on. Assuch, the node 183 is electrically connected to the ground Vss 111, andhas a voltage close to 0V. As such, the transistor 165 is turned off,and the node 186 is electrically connected to the ground Vss 111 and hasa voltage 0V. Accordingly, the transistor 166 is turned off, and thenode 183 is electrically connected to the ground Vss 111 and has avoltage 0V. In this case, the capacitor 122 is charged by the powersupply pin VDD 102 via the transistor 142. In this example, thetransistor 142 is a diode-connected HEMT used as a rectifying diode,which naturally has a forward voltage (Vf). That is, the voltage at thenode 184 will maximally be charged to 6V−Vf. In a first example,assuming the forward voltages and threshold voltages of all transistorsin FIG. 1 are equal to 1.5V, the maximum voltage at the node 184 whenthe circuit 100 is turned off is 6V−1.5V=4.5V.

When the circuit 100 is turned on and the Vin has a voltage of 6V, thetransistor 153 is turned on and the transistor 156 is turned off. Thenode 181 has the same voltage 0V as the ground Vss 111, which enablesthe transistors 161, 162, 163 to be turned off. As such, the node 185 iselectrically connected to the node 184, and has a same voltage as thenode 184. This induces the transistor 165 to be turned on, which enablesthe node 186 to be charged by the voltage at the node 184. This in turninduces the transistor 166 to be turned on, which enables the node 183to be charged by the power supply pin VDD 102. As such, the voltage atthe node 183 can maximally be charged to 6V, same as the voltage of thepower supply pin VDD 102. Based on the 4.5V voltage difference stored bythe capacitor 122 when the circuit 100 is off, the voltage at the node184 can maximally be charged and increased to 6V+4.5V=10.5V, i.e. thevoltage at the node 184 is boot-strapped to 10.5V. Accordingly, the node185, which is electrically connected to both the source and the gate ofthe transistor 164, is charged to 10.5V as well.

While the node 186 is also charged by the voltage 10.5V at the node 184,the voltage of the node 186 cannot reach 10.5V. Because the node 186 iselectrically connected to the source of the transistor 165, to keep thetransistor 165 on, the gate source voltage difference Vgs of thetransistor 165 must be larger than the threshold voltage (Vt) of thetransistor 165. As it is assumed Vt=1.5V in the first example, themaximum voltage the node 186 can reach in the first example when thecircuit 100 is turned on is 10.5V−Vt=10.5V−1.5V=9V. As such, anenhancement-mode high-electron-mobility transistor (E-HEMT) pull-updevice consumes at least one Vt voltage drop.

The node 182 is electrically connected to the node 181 and has a samevoltage as that of the node 181. That is, when the circuit 100 is turnedoff, the node 182 has the voltage 6V; when the circuit 100 is turned on,the node 182 has the voltage 0V. When the circuit 100 is turned off, the6V voltage at the node 182 enables the transistors 171, 172 to be turnedon. As such, the node 187 is electrically connected to the ground Vss111, and has a voltage 0V. Here, the transistor 173 is turned off due tothe 0V voltage at the node 186 when the circuit 100 is turned off asdiscussed above. Because the node 187 has the voltage 0V, the transistor174 is turned off, and the node 188 is electrically connected to theground Vss 111 and has a voltage 0V. In this case, the capacitor 123 ischarged by the power supply pin VDD 103 via the transistor 143. In thisexample, the transistor 143 is a diode-connected HEMT used as arectifying diode, which naturally has a forward voltage (Vf). That is,the voltage at the node 189 will maximally be charged to 6V−Vf. In thefirst example, assuming the forward voltages and threshold voltages ofall transistors in FIG. 1 are equal to 1.5V, the maximum voltage at thenode 189 when the circuit 100 is turned off is 6V−1.5V=4.5V.

When the circuit 100 is turned on, the node 182, like the node 181, hasthe same voltage 0V as the ground Vss 111, which enables the transistors171, 172 to be turned off. As discussed above, the node 186, which iselectrically connected to the gate of the transistor 173, has a maximumvoltage of 9V when the circuit 100 is turned on. As such, the transistor173 is turned on and the node 187 is charged by the node 189. Thisinduces the transistor 174 to be turned on, which enables the node 188to be charged by the power supply pin VDD 103. As such, the voltage atthe node 188 can maximally be charged to 6V, same as the voltage of thepower supply pin VDD 102. Based on the 4.5V voltage difference stored bythe capacitor 123 when the circuit 100 is off, the voltage at the node189 can maximally be charged and increased to 6V+4.5V=10.5V, i.e. thevoltage at the node 189 is boot-strapped to 10.5V.

While the node 187 is charged by the voltage 10.5V at the node 189, thevoltage of the node 187 cannot reach 10.5V. Because the node 187 iselectrically connected to the source of the transistor 173, to keep thetransistor 173 on, the gate source voltage difference Vgs of thetransistor 173 must be larger than the threshold voltage (Vt) of thetransistor 173. The gate of the transistor 173 is electrically connectedto the node 186, which has a maximum voltage 9V when the circuit 100 isturned on. As it is assumed Vt=1.5V in the first example, the maximumvoltage the node 187 can reach in the first example when the circuit 100is turned on is 9V−Vt=9V−1.5V=7.5V. Now the transistor 174 has a gatesource voltage difference Vgs=7.5V−6V=1.5V, which is exactly equal tothe threshold voltage Vt=1.5V of the transistor 174. This leaves novoltage margin at the last stage of the multi-stage boot-strappeddriver. That is, in the first example where Vf=Vt=1.5V, there is notenough over-drive voltage to drive the power switch HEMT 175. Even ifthe power switch HEMT 175 can be driven, it would be significantly slowas the current flowing through the transistor 174 and the node 188 wouldbe very slow due to no Vgs margin compared to the Vt. The aboveconclusion has not even taken into consideration of the Vt variation(e.g. 3-σ variation of 0.5V), which typically exists in all processtechnologies. After counting the 3-σ variation of 0.5V, the circuit 100,under the Vt=1.5V assumption, may not be able to drive the power switchHEMT 175 at all.

In a second example, it is assumed the forward voltages and thresholdvoltages of all transistors in FIG. 1 are equal to 1V. In this case,when the circuit 100 is turned off, the node 181 has the same voltage6V, which enables the transistors 161, 162, 163 to be turned on. Assuch, the node 185 is electrically connected to the ground Vss 111 andhas a voltage 0V. As such, the transistor 165 is turned off, and thenode 186 is electrically connected to the ground Vss 111 and has avoltage 0V. Accordingly, the transistor 166 is turned off, and the node183 is electrically connected to the ground Vss 111 and has a voltage0V. The capacitor 122 is charged by the power supply pin VDD 102 via thetransistor 142. Because the transistor 142 is a diode-connected HEMTused as a rectifying diode which naturally has a forward voltage (Vf),the node 184 can have a maximum voltage of 6V−Vf=6V−1V=5V.

When the circuit 100 is turned on, the node 181 has the same voltage 0Vas the ground Vss 111, which enables the transistors 161, 162, 163 to beturned off. As such, the node 185 is electrically connected to the node184, and has a same voltage as the node 184. This induces the transistor165 to be turned on, which enables the node 186 to be charged by thevoltage at the node 184. This in turn induces the transistor 166 to beturned on, which enables the node 183 to be charged by the power supplypin VDD 102. As such, the node 183 has a maximum voltage of 6V, same asthe voltage of the power supply pin VDD 102. Based on the 5V voltagedifference stored by the capacitor 122 when the circuit 100 is off, thevoltage at the node 184 can maximally be charged and increased to6V+5V=11V, i.e. the voltage at the node 184 is boot-strapped to 11V.Accordingly, the node 185, which is electrically connected to both thesource and the gate of the transistor 164, is charged to 11V as well.While the node 186 is also charged by the voltage 11V at the node 184,the voltage of the node 186 cannot reach 11V. Because the node 186 iselectrically connected to the source of the transistor 165, to keep thetransistor 165 on, the gate source voltage difference Vgs of thetransistor 165 must be larger than the threshold voltage (Vt) of thetransistor 165. As it is assumed Vt=1V in the second example, themaximum voltage the node 186 can reach in the second example when thecircuit 100 is turned on is 11V−Vt=11V−1V=10V.

The node 182 is electrically connected to the node 181 and has a samevoltage as that of the node 181. That is, when the circuit 100 is turnedoff, the node 182 has the voltage 6V; when the circuit 100 is turned on,the node 182 has the voltage 0V. When the circuit 100 is turned off, the6V voltage at the node 182 enables the transistors 171, 172 to be turnedon. As such, the node 187 is electrically connected to the ground Vss111, and has a voltage 0V. Here, the transistor 173 is turned off due tothe 0V voltage at the node 186 when the circuit 100 is turned off asdiscussed above. Because the node 187 has the voltage 0V, the transistor174 is turned off, and the node 188 is electrically connected to theground Vss 111 and has a voltage 0V. In this case, the capacitor 123 ischarged by the power supply pin VDD 103 via the transistor 143. Becausethe transistor 143 is a diode-connected HEMT used as a rectifying diodewhich naturally has a forward voltage (Vf), the node 189 has a maximumvoltage of 6V−Vf=6V-−V=5V.

When the circuit 100 is turned on, the node 182, like the node 181, hasthe same voltage 0V as the ground Vss 111, which enables the transistors171, 172 to be turned off. As discussed above, the node 186, which iselectrically connected to the gate of the transistor 173, has a maximumvoltage of 10V when the circuit 100 is turned on. As such, thetransistor 173 is turned on and the node 187 is charged by the node 189.This induces the transistor 174 to be turned on, which enables the node188 to be charged by the power supply pin VDD 103. As such, the voltageat the node 188 can maximally be charged to 6V, same as the voltage ofthe power supply pin VDD 102. Based on the 5V voltage difference storedby the capacitor 123 when the circuit 100 is off, the voltage at thenode 189 can maximally be charged and increased to 6+5V=11V, i.e. thevoltage at the node 189 is boot-strapped to 11V.

While the node 187 is charged by the voltage 11V at the node 189, thevoltage of the node 187 cannot reach 11V. Because the node 187 iselectrically connected to the source of the transistor 173, to keep thetransistor 173 on, the gate source voltage difference Vgs of thetransistor 173 must be larger than the threshold voltage (Vt) of thetransistor 173. The gate of the transistor 173 is electrically connectedto the node 186, which has a maximum voltage 10V when the circuit 100 isturned on. As it is assumed Vt=1V in the second example, the maximumvoltage the node 187 can reach in the second example when the circuit100 is turned on is 10−Vt=10V−1V=9V. Now the transistor 174 has a gatesource voltage difference Vgs=9V−6V=3V, which is much larger than thethreshold voltage Vt=1V of the transistor 174. This leaves enoughvoltage margin at the last stage of the multi-stage boot-strappeddriver. That is, in the second example where Vf=Vt=1V, there is enoughover-drive voltage to drive the power switch HEMT 175. However, sinceall transistors, including the power switch HEMT 175, in FIG. 1 areusing a same Vt, a reduced Vt at the power switch HEMT 175 may cause thenoise immunity of the output power switch 175 become significantly worsedue to not being able to withstand a large back-feed-through impulse(di/dt) voltage to the gate of the output power switch 175. Becausethere is inevitable parasitic capacitance between the drain and the gateof the power switch HEMT 175, a voltage impulse will feed back from thedrain of the power switch HEMT 175 to the gate of the power switch HEMT175 through the parasitic capacitance. This could accidently turn on thepower switch HEMT 175 so long as the noise voltage is larger than thereduced Vt of the power switch HEMT 175, even when the circuit 100 isturned off.

As such, in a third example, the forward voltages and threshold voltagesof all transistors in FIG. 1 are not all the same. In the third example,it is assumed that the transistors 142, 143, 165, 166, 173, 174 have asmaller Vt of 1V, while the other transistors in FIG. 1 have a larger Vtof 1.5V. In this case, when the circuit 100 is turned off, the node 181has the same voltage 6V, which enables the transistors 161, 162, 163 tobe turned on. As such, the node 185 is electrically connected to theground Vss 111 and has a voltage 0V. As such, the transistor 165 isturned off, and the node 186 is electrically connected to the ground Vss111 and has a voltage 0V. Accordingly, the transistor 166 is turned off,and the node 183 is electrically connected to the ground Vss 111 and hasa voltage 0V. The capacitor 122 is charged by the power supply pin VDD102 via the transistor 142. Because the transistor 142 has a forwardvoltage Vf equal to its Vt, the node 184 can have a maximum voltage of6V−Vf=6V−1V=5V.

When the circuit 100 is turned on, the node 181 has the same voltage 0Vas the ground Vss 111, which enables the transistors 161, 162, 163 to beturned off. As such, the node 185 is electrically connected to the node184, and has a same voltage as the node 184. This induces the transistor165 to be turned on, which enables the node 186 to be charged by thevoltage at the node 184. This in turn induces the transistor 166 to beturned on, which enables the node 183 to be charged by the power supplypin VDD 102. As such, the node 183 has a maximum voltage of 6V, same asthe voltage of the power supply pin VDD 102. Based on the 5V voltagedifference stored by the capacitor 122 when the circuit 100 is off, thevoltage at the node 184 can maximally be charged and increased to6V+5V=11V, i.e. the voltage at the node 184 is boot-strapped to 11V.Accordingly, the node 185, which is electrically connected to both thesource and the gate of the transistor 164, is charged to 11V as well.While the node 186 is also charged by the voltage 11V at the node 184,the voltage of the node 186 cannot reach 11V. Because the node 186 iselectrically connected to the source of the transistor 165, to keep thetransistor 165 on, the gate source voltage difference Vgs of thetransistor 165 must be larger than the Vt=1V of the transistor 165. Sothe maximum voltage the node 186 can reach in the third example when thecircuit 100 is turned on is 11V−1V=10V.

The node 182 is electrically connected to the node 181 and has a samevoltage as that of the node 181. That is, when the circuit 100 is turnedoff, the node 182 has the voltage 6V; when the circuit 100 is turned on,the node 182 has the voltage 0V. When the circuit 100 is turned off, the6V voltage at the node 182 enables the transistors 171, 172 to be turnedon. As such, the node 187 is electrically connected to the ground Vss111, and has a voltage 0V. Here, the transistor 173 is turned off due tothe 0V voltage at the node 186 when the circuit 100 is turned off asdiscussed above. Because the node 187 has the voltage 0V, the transistor174 is turned off, and the node 188 is electrically connected to theground Vss 111 and has a voltage 0V. In this case, the capacitor 123 ischarged by the power supply pin VDD 103 via the diode-connectedtransistor 143. Because the diode-connected transistor 143 has a forwardvoltage Vf equal to its Vt, the node 189 has a maximum voltage of6V−Vf=6V−1V=5V.

When the circuit 100 is turned on, the node 182, like the node 181, hasthe same voltage 0V as the ground Vss 111, which enables the transistors171, 172 to be turned off. As discussed above, the node 186, which iselectrically connected to the gate of the transistor 173, has a maximumvoltage of 10V when the circuit 100 is turned on. As such, thetransistor 173 is turned on and the node 187 is charged by the node 189.This induces the transistor 174 to be turned on, which enables the node188 to be charged by the power supply pin VDD 103. As such, the voltageat the node 188 can maximally be charged to 6V, same as the voltage ofthe power supply pin VDD 102. Based on the 5V voltage difference storedby the capacitor 123 when the circuit 100 is off, the voltage at thenode 189 can maximally be charged and increased to 6V+5V=11V, i.e. thevoltage at the node 189 is boot-strapped to 11V.

While the node 187 is charged by the voltage 11V at the node 189, thevoltage of the node 187 cannot reach 11V. Because the node 187 iselectrically connected to the source of the transistor 173, to keep thetransistor 173 on, the gate source voltage difference Vgs of thetransistor 173 must be larger than the threshold voltage Vt=1V of thetransistor 173. Because the gate of the transistor 173 is electricallyconnected to the node 186, which has a maximum voltage 10V when thecircuit 100 is turned on, the maximum voltage the node 187 can reach inthe third example when the circuit 100 is turned on is 10V−Vt=10V−1V=9V.Now the transistor 174 has a gate source voltage differenceVgs=9V−6V=3V, which is much larger than the threshold voltage Vt=1V ofthe transistor 174. This leaves enough voltage margin at the last stageof the multi-stage boot-strapped driver. That is, in the third examplewhere the transistors 142, 143, 165, 166, 173, 174 have a smaller Vt=1V,there is enough over-drive voltage to drive the power switch HEMT 175.In addition, since all other transistors, including the power switchHEMT 175, in FIG. 1 are having a larger Vt=1.5V, the noise immunity ofthe output power switch 175 will be better than the second example,because a larger Vt of the power switch HEMT 175 can significantlywithstand impulse voltage noise fed back from the drain of the powerswitch HEMT 175 to the gate of the power switch HEMT 175.

In various embodiments, the power switch HEMT 175 may have an evenlarger Vt like 2V. The disclosed circuit design for dual-Vt or multi-Vttransistors can reduce both Vt of the pull-up E-HEMT transistors and Vfof the diode-connected E-HEMT rectifiers of the multi-stage driver toprovide enough over-drive voltage and dramatically reduce staticcurrent, without compromising the noise immunity of the output powerswitch. To use dual-Vt or multi-Vt transistors in a same IC, differentgate stack materials can be used for different transistors formed on asame wafer. For example, a transistor having a pGaN gate stack dopedwith Mg will have a higher Vt than a transistor having a pGaN gate stackdoped with Be.

FIG. 2 illustrates a cross-sectional view of an exemplary semiconductordevice 200 including transistors with different gate stack materials, inaccordance with some embodiments of the present disclosure. As shown inFIG. 2, the semiconductor device 200 in this example includes a siliconlayer 210 and a transition layer 220 disposed on the silicon layer 210.The semiconductor device 200 further includes a first layer 230comprising a first III-V semiconductor material formed over thetransition layer 220.

The semiconductor device 200 further includes a second layer 240 (apolarization layer) comprising a second III-V semiconductor materialdisposed on the first layer 230. The second III-V semiconductor materialis different from the first III-V semiconductor material. For example,the first III-V semiconductor material may be gallium nitride (GaN);while the second III-V semiconductor material may be aluminum galliumnitride (AlGaN).

As shown in FIG. 2, the semiconductor device 200 further includes afirst transistor 201 and a second transistor 202 formed over the firstlayer 230. The first transistor 201 comprises a first gate structure251, a first source region 281 and a first drain region 291. The secondtransistor 202 comprises a second gate structure 252, a second sourceregion 282 and a second drain region 292.

The semiconductor device 200 further includes a polarization modulationlayer 241, 242 disposed on the second layer 240, and a passivation layer250 disposed partially on the polarization modulation layer andpartially on the second layer 240. The polarization modulation layer hasa first polarization modulation portion 241 under the first gatestructure 251, and a second polarization modulation portion 242 underthe second gate structure 252. Each of the first polarization modulationportion 241 and the second polarization modulation portion 242 has ap-type doped GaN (pGaN). In one embodiment, the first polarizationmodulation portion 241 and the second polarization modulation portion242 include different p-type doping materials to induce differentthreshold voltages for the first transistor 201 and the secondtransistor 202.

The sources 281, 282 and the drains 291, 292 of the two transistors 201,202 are formed through the second layer 240 and the passivation layer250, and disposed on the first layer 230. The first gate structure 251is disposed on the pGaN portion 241 and between the first source region281 and the first drain region 291. The second gate structure 252 isdisposed on the pGaN portion 242 and between the second source region282 and the second drain region 292.

In one embodiment, the first transistor 201 and the second transistor202 are high electron mobility transistors to be used in a samemulti-stage driver circuit. For example, the first transistor 201 isused as a power switch transistor and has a first threshold voltage. Thesecond transistor 202 is used as a driver transistor and has a secondthreshold voltage that is lower than the first threshold voltage.Accordingly, the p-type doping material of the first polarizationmodulation portion 241 has a lower work-function than the p-type dopingmaterial of the second polarization modulation portion 242. For example,the first polarization modulation portion 241 is doped with Mg, whilethe second polarization modulation portion 242 is doped with Be.

In addition, the semiconductor device 200 includes an interlayerdielectric (ILD) layer 260 disposed partially on the passivation layer250 and partially on the first transistor 201 and the second transistor202. The semiconductor device 200 also includes metal contacts 271disposed on and in contact with the sources 281, 282 and the drains 291,292 respectively, and includes a first metal layer 272 on the metalcontacts 271.

FIGS. 3A, 3B, 3C, 3D, 3E, 3F, 3G, 3H, 3I, 3J, 3K, 3L, 3M, 3N, 3O and 3Pillustrate cross-sectional views of an exemplary semiconductor deviceduring various fabrication stages, in accordance with some embodimentsof the present disclosure. In some embodiments, the semiconductor devicemay be included in an integrated circuit (IC). In addition, FIGS. 3Athrough 3P are simplified for a better understanding of the concepts ofthe present disclosure. For example, although the figures illustrate twotransistors, it is understood the semiconductor device may include morethan two transistors, and the IC may include a number of other devicescomprising resistors, capacitors, inductors, fuses, etc., which are notshown in FIGS. 3A through 3P, for purposes of clarity of illustration.

FIG. 3A is a cross-sectional view of the semiconductor device includinga substrate 310, which is provided at one of the various stages offabrication, according to some embodiments of the present disclosure.The substrate 310 may be formed of silicon, as shown in FIG. 3A, oranother semiconductor material.

FIG. 3B is a cross-sectional view of the semiconductor device includinga transition or buffer layer 320, which is formed on the substrate 310at one of the various stages of fabrication, according to someembodiments of the present disclosure. The transition or buffer layer320 may be formed by epitaxial growth. According to various embodiments,the transition or buffer layer 320 includes a nucleation layer ofaluminum nitride (AlN) and serves as a buffer to reduce the stressbetween the substrate 310 and the layer on top of the transition orbuffer layer 320. In one embodiment, the transition or buffer layer 320and the operation step shown in FIG. 3B is optional and can be removed.

FIG. 3C is a cross-sectional view of the semiconductor device includinga first III-V semiconductor material layer 330, which is formedoptionally on the transition or buffer layer 320 or directly on thesubstrate 310 at one of the various stages of fabrication, according tosome embodiments of the present disclosure. The first III-Vsemiconductor material layer 330 may be formed by epitaxial growth.According to various embodiments, the first III-V semiconductor materiallayer 330 includes a gallium nitride (GaN). When the first III-Vsemiconductor material layer 330 is formed on the transition or bufferlayer 320, the transition or buffer layer 320 can reduce the stressbetween the substrate 310 and the first III-V semiconductor materiallayer 330. After transistors are formed over the first III-Vsemiconductor material layer 330, the first III-V semiconductor materiallayer 330 serves as a channel layer for the transistors.

FIG. 3D is a cross-sectional view of the semiconductor device includinga second III-V semiconductor material layer 340, which is formed on thefirst III-V semiconductor material layer 330 at one of the variousstages of fabrication, according to some embodiments of the presentdisclosure. The second III-V semiconductor material layer 340 may beformed by epitaxial growth. According to various embodiments, the secondIII-V semiconductor material layer 340 includes an aluminum galliumnitride (AlGaN). After transistors are formed over the first III-Vsemiconductor material layer 330 and the second III-V semiconductormaterial layer 340, a 2-dimensional electron gas (2-DEG) will be formedat the interface between the first III-V semiconductor material layer330 and the second semiconductor material layer 340.

FIG. 3E is a cross-sectional view of the semiconductor device includinga p-type doped GaN (pGaN) gate stack 341, which is formed on the secondIII-V semiconductor material layer 340 at one of the various stages offabrication, according to some embodiments of the present disclosure.The pGaN gate stack 341 is an island region comprising a first dopingmaterial, which may be one of the column I and column II elements, e.g.Mg, Li, Na, Be, Ca, etc.

FIG. 3F is a cross-sectional view of the semiconductor device includinga mask 345 covering and protecting the pGaN gate stack 341, which isformed on the second III-V semiconductor material layer 340 at one ofthe various stages of fabrication, according to some embodiments of thepresent disclosure. FIG. 3G is a cross-sectional view of thesemiconductor device including a patterned mask 345, which is formed onthe second III-V semiconductor material layer 340 at one of the variousstages of fabrication, according to some embodiments of the presentdisclosure. At this stage, the patterned mask 345 has a pattern todefine an opening 346 to expose a portion of the second III-Vsemiconductor material layer 340.

FIG. 3H is a cross-sectional view of the semiconductor device includinga pGaN gate stack 342, which is deposited and polished in the opening346 at one of the various stages of fabrication, according to someembodiments of the present disclosure. The pGaN gate stack 342 is anisland region comprising a second doping material, which may be one ofthe column I and column II elements, e.g. Mg, Li, Na, Be, Ca, etc. Thesecond doping material in the pGaN gate stack 342 is different from thefirst doping material in the pGaN gate stack 341. The pGaN gate stacks341, 342 form a polarization modulation layer, which modulates thedipole concentration in the AlGaN layer 340 to result in changing the2-DEG concentration in the AlGaN/GaN interface channel.

FIG. 3I is a cross-sectional view of the semiconductor device includinga passivation layer 350, which is formed on the second III-Vsemiconductor material layer 340, and the polarization modulation layer341, 342 at one of the various stages of fabrication, according to someembodiments of the present disclosure. The passivation layer 350 isformed over the second III-V semiconductor material layer 340 and overthe pGaN gate stacks 341, 342. According to various embodiments, thepassivation layer 350 is formed using a deposition procedure (e.g.,chemical deposition, physical deposition, etc.). The passivation layer350 may comprise silicon oxide, silicon nitride, silicon oxynitride,carbon doped silicon oxide, carbon doped silicon nitride, carbon dopedsilicon oxynitride, zinc oxide, zirconium oxide, hafnium oxide, titaniumoxide, or another suitable material. In one embodiment, after depositingthe passivation layer 350, the passivation layer 350 undergoes apolishing and/or etching procedure. The polishing and/or etchingprocedure includes, e.g. a chemical-mechanical planarization (CMP)(i.e., chemical-mechanical polishing) process that is used to polish thesurface of the passivation layer 350 and remove topographicalirregularities.

FIG. 3J is a cross-sectional view of the semiconductor device includingsource and drain contacts 381, 391, 382, 392, which are formed throughthe second III-V semiconductor material layer 340 and the passivationlayer 350 and disposed on the first III-V semiconductor material layer330 at one of the various stages of fabrication, according to someembodiments of the present disclosure. The source and drain contacts maybe formed as non-rectifying electrical junctions, i.e. ohmic contacts.

FIG. 3K is a cross-sectional view of the semiconductor device includinga mask 355, which is formed on the passivation layer 350 at one of thevarious stages of fabrication, according to some embodiments of thepresent disclosure. At this stage, the mask 355 has a pattern to exposeportions of the passivation layer 350 on top of the pGaN gate stacks341, 342. As such, a first opening 357 is formed on the pGaN gate stack341 between the first pair of source 381 and drain 391 by etching thepassivation layer 350 with the patterned mask 355; a second opening 358is formed on the pGaN gate stack 342 between the second pair of source382 and drain 392 by etching the passivation layer 350 with thepatterned mask 355.

FIG. 3L is a cross-sectional view of the semiconductor device includinga first gate 351 and a second gate 352, which are deposited and polishedin the first opening 357 and the second opening 358 respectively at oneof the various stages of fabrication, according to some embodiments ofthe present disclosure. According to various embodiments, the first gate351 and the second gate 352 may be formed of metal materials like:tungsten (W), nickel (Ni), titanium/tungsten/titanium-nitride (Ti/W/TiN)metal stack, or titanium/nickel/titanium-nitride (Ti/Ni/TiN) metalstack.

FIG. 3M is a cross-sectional view of the semiconductor device, where themask 355 is removed from the passivation layer 350 after the metal gatesare formed, at one of the various stages of fabrication, according tosome embodiments of the present disclosure. After the mask 355 isremoved, each of the source regions 381, 382, the drain regions 391,392, and the gate structures 351, 352 has an exposed portion on top ofthe passivation layer 350.

FIG. 3N is a cross-sectional view of the semiconductor device includingan interlayer dielectric (ILD) layer 360, which is formed on thepassivation layer 350, at one of the various stages of fabrication,according to some embodiments of the present disclosure. The ILD layer360 covers the passivation layer 350 and the exposed portions of thesource regions 381, 382, the drain regions 391, 392, and the gatestructures 351, 352 that are formed at the stage shown in FIG. 3M. TheILD layer 360 is formed of a dielectric material and may be patternedwith holes for metal interconnects or contacts for the source and draincontacts 381, 382, 391, 392 as well as the gate structures 351, 352.

FIG. 3O is a cross-sectional view of the semiconductor device includingmetal contacts 371, each of which is formed on a source or draincontact, at one of the various stages of fabrication, according to someembodiments of the present disclosure. As discussed above, the ILD layer360 is patterned with holes each of which is on one of the source anddrain contacts 381, 382, 391, 392. As such, the metal contacts 371 canbe formed in these holes to be in contact with the source and draincontacts 381, 382, 391, 392, respectively.

FIG. 3P is a cross-sectional view of the semiconductor device includinga first metal layer 372, which is formed on the metal contacts 371, atone of the various stages of fabrication, according to some embodimentsof the present disclosure. The first metal layer 372 includes metalmaterial and is formed over the ILD layer 360 and in contact with themetal contacts 371.

FIG. 4A and FIG. 4B show a flow chart illustrating an exemplary method400 for forming a semiconductor device including transistors withdifferent gate stack materials, in accordance with some embodiments ofthe present disclosure. As shown in FIG. 4A, at operation 402, atransition/buffer layer is formed on a semiconductor substrate byepitaxial growth. A GaN layer is formed at operation 404 on thetransition/buffer layer by epitaxial growth. At operation 406, an AlGaNlayer is formed on the GaN layer by epitaxial growth. At operation 408,a first polarization modulation portion having a first p-type dopant isdeposited and defined on the AlGaN layer. At operation 410, a mask isput on the first polarization modulation portion and the AlGaN layer. Atoperation 412, an opening is defined on the mask for a secondpolarization modulation portion. At operation 414, the secondpolarization modulation portion having a second p-type dopant isdeposited and polished in the opening. At operation 415, a passivationlayer is deposited and polished on the polarization modulation portionsand AlGaN layer. The process then goes to the operation 416 in FIG. 4B.

Source and drain ohmic contacts are formed at operation 416 through thepassivation layer and the AlGaN layer. At operation 418, openings aredefined for metal gate areas on the polarization modulation portions byetching with a mask. At operation 420, the metal gate material isdeposited and polished in the openings to form the gates. At operation422, the mask on the passivation layer is removed. At operation 424, adielectric layer is deposited and polished on the sources, drains, gatesand the passivation layer. Metal contacts are formed and defined atoperation 426 on the sources, drains, and gates. At operation 428, afirst metal layer is formed and defined on the dielectric layer and themetal contacts. The order of the operations shown in FIG. 4A and FIG. 4Bmay be changed according to different embodiments of the presentdisclosure.

In an embodiment, a semiconductor structure is disclosed. Thesemiconductor structure includes: a substrate; a channel layer formedover the substrate; a first transistor formed over the channel layer,wherein the first transistor comprises a first source region, a firstdrain region, a first gate structure, and a first polarizationmodulation portion under the first gate structure; and a secondtransistor formed over the channel layer, wherein the second transistorcomprises a second source region, a second drain region, a second gatestructure, and a second polarization modulation portion under the secondgate structure, wherein the first polarization modulation portion ismade of a material different from that of the second polarizationmodulation portion.

In another embodiment, a circuit is disclosed. The circuit includes: afirst transistor including a first gate, a first source, a first drainand a first polarization modulation portion under the first gate; and asecond transistor including a second gate, a second source, a seconddrain and a second polarization modulation portion under the secondgate. The first transistor and the second transistor are formed on asame semiconductor wafer. The first polarization modulation portion ismade of a material different from that of the second polarizationmodulation portion.

In yet another embodiment, a method for forming a semiconductorstructure is disclosed. The method includes: forming a channel layerover a substrate; forming a first transistor over the channel layer,wherein the first transistor comprises a first source region, a firstdrain region, a first gate structure, and a first polarizationmodulation portion under the first gate structure; and forming a secondtransistor over the channel layer. The second transistor comprises asecond source region, a second drain region, a second gate structure,and a second polarization modulation portion under the second gatestructure. The first polarization modulation portion is made of amaterial different from that of the second polarization modulationportion.

The foregoing outlines features of several embodiments so that thoseordinary skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art should appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodimentsintroduced herein. Those skilled in the art should also realize thatsuch equivalent constructions do not depart from the spirit and scope ofthe present disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A semiconductor structure, comprising: a substrate; a channel layer formed over the substrate; a first transistor formed over the channel layer, wherein the first transistor comprises a first source region, a first drain region, a first gate structure, and a first polarization modulation portion under the first gate structure and over the channel layer; and a second transistor formed over the channel layer, wherein the second transistor comprises a second source region, a second drain region, a second gate structure, and a second polarization modulation portion under the second gate structure and over the channel layer, wherein the first polarization modulation portion and the second polarization modulation portion are arranged horizontally in a same polarization modulation layer, and the first polarization modulation portion is made of a material different from that of the second polarization modulation portion.
 2. The semiconductor structure of claim 1, wherein: the first transistor and the second transistor are high electron mobility transistors to be used in a same multi-stage driver circuit.
 3. The semiconductor structure of claim 1, wherein: the first transistor has a first threshold voltage; and the second transistor has a second threshold voltage that is lower than the first threshold voltage.
 4. The semiconductor structure of claim 3, wherein: the first polarization modulation portion comprises gallium nitride (GaN) doped by a first doping material; the second polarization modulation portion comprises GaN doped by a second doping material; and the first polarization modulation portion has a different work function than the second polarization modulation portion.
 5. The semiconductor structure of claim 4, wherein: the first doping material is magnesium (Mg); and the second doping material is beryllium (Be).
 6. The semiconductor structure of claim 1, wherein: each of the first polarization modulation portion and the second polarization modulation portion comprises gallium nitride (GaN) doped by at least one p-type doping material selected from column I and column II elements.
 7. The semiconductor structure of claim 1, further comprising an active layer disposed on the channel layer, wherein: the channel layer comprises a first III-V semiconductor material; and the active layer comprises a second III-V semiconductor material that is different from the first III-V semiconductor material.
 8. The semiconductor structure of claim 7, wherein: the first III-V semiconductor material comprises gallium nitride (GaN); and the second III-V semiconductor material comprises aluminum gallium nitride (AlGaN).
 9. A circuit, comprising: a first transistor including a first gate, a first source, a first drain and a first polarization modulation portion that is under and in contact with the first gate; and a second transistor including a second gate, a second source, a second drain and a second polarization modulation portion that is under and in contact with the second gate, wherein: the first transistor and the second transistor are formed on a same semiconductor wafer, the first polarization modulation portion and the second polarization modulation portion are arranged horizontally in a same polarization modulation layer, and the first polarization modulation portion is made of a material different from that of the second polarization modulation portion.
 10. The circuit of claim 9, wherein: the first transistor has a first threshold voltage; and the second transistor has a second threshold voltage that is different from the first threshold voltage.
 11. The circuit of claim 9, wherein: at least one of the first source and the first drain is electrically connected to a ground voltage; and at least one of the second source and the second drain is electrically connected to a positive supply voltage.
 12. The circuit of claim 11, wherein: the first threshold voltage is different from the second threshold voltage.
 13. The circuit of claim 12, wherein: the first polarization modulation portion comprises gallium nitride (GaN) doped by a first doping material; the second polarization modulation portion comprises GaN doped by a second doping material; and the first polarization modulation portion has a lower work function than the second polarization modulation portion.
 14. The circuit of claim 12, wherein: at least one of the first source and the first drain is electrically connected to an output pin of the circuit.
 15. The circuit of claim 12, wherein the first transistor is at least one of: a high voltage enhancement-mode high electron mobility transistor (HV E-HEMT); a low voltage enhancement-mode high electron mobility transistor (LV E-HEMT); and a low voltage depletion-mode high electron mobility transistor (LV D-HEMT).
 16. The circuit of claim 12, wherein the second transistor is a low voltage enhancement-mode high electron mobility transistor (LV E-HEMT).
 17. The circuit of claim 9, wherein the first gate is physically coupled to the second source.
 18. A method for forming a semiconductor structure, comprising: forming a channel layer over a substrate; forming a first transistor over the channel layer, wherein the first transistor comprises a first source region, a first drain region, a first gate structure, and a first polarization modulation portion under the first gate structure and over the channel layer; and forming a second transistor over the channel layer, wherein the second transistor comprises a second source region, a second drain region, a second gate structure, and a second polarization modulation portion under the second gate structure and over the channel layer, wherein the first polarization modulation portion and the second polarization modulation portion are formed horizontally in a same polarization modulation layer, and the first polarization modulation portion is made of a material different from that of the second polarization modulation portion.
 19. The method of claim 18, wherein: the first transistor and the second transistor are high electron mobility transistors to be used in a same multi-stage driver circuit; the first transistor has a first threshold voltage; the second transistor has a second threshold voltage that is different from the first threshold voltage; and the first polarization modulation portion is made of a material having a different work function than that of a material of the second polarization modulation portion.
 20. The method of claim 18, further comprising: forming an active layer on the channel layer; defining a first opening on a mask covering the active layer; forming the first polarization modulation portion in the first opening; defining a second opening on the mask; and forming the second polarization modulation portion in the second opening. 